Si(1-x)Gex single-crystal thin films have been grown epitaxially on silicon. However, the thickness of these films is limited because, beyond a certain thickness (referred to as the critical thickness), the strain induced in the Si(1-x)Gex by the lattice mismatch between the growth substrate and the Si(1-x)Gex begins to plastically relax, which results in the formation of lattice mismatch-induced misfit dislocations in the Si(1-x)Gex. Moreover, as the Ge content of the Si(1-x)Gex increases, the critical thickness of the film decreases. As a result, high-quality, single-crystalline films of Si(1-x)Gex with a high Ge content cannot be grown on silicon to adequate thicknesses for many practical processing techniques and device applications. High-Ge-content Si(1-x)Gex single-crystal thin films can be grown epitaxially on germanium. However, the critical thickness of these films decreases with increasing silicon content and, therefore, high-quality Si(1-x)Gex films with thicknesses useful for many device applications can only be achieved for Si(1-x)Gex films with a very high Ge content when germanium is used as the growth substrate.
High Ge content Si(1-x)Gex has been grown epitaxially over compositionally graded, plastically relaxed growth substrates. Unfortunately, the plastically relaxed growth substrates are characterized by lattice mismatch-induced misfit dislocations and their associated threading dislocations, and these propagate through the Si(1-x)Gex. Moreover, the resulting Si(1-x)Gex films are characterized by a non-uniform strain distribution and small-angle tilt boundaries, which degrade the crystal quality and, therefore, the performance of devices incorporating the Si(1-x)Gex.